Strengths polymers (also referred to as strength resins, strength aids, strength additives and the like) are extensively utilized in paper manufacture. It is often distinguished between dry strength polymers and wet strength polymers, though dry strength polymers often impart a certain degree of wet strength to the paper, and vice versa. Today, the most common types of synthetic dry and/or wet strength polymers are based on polyvinylamine or polyacrylamide. Other resins such as polyvinyl alcohol or lattices are used, but generally these are seen in surface applications to the paper, rather than as wet-end additives. Similarly, styrene acrylic resins have shown superior performance when applied through surface application rather than as stock additives.
A number of polymers are commercially available as dry or wet strength polymers. They can be classified in the following three categories:    (i) Polymers capable of only forming hydrogen bonds to starch and/or cellulose fibers, such as certain polyacrylamides, usually do not provide significant degrees of wet strength but can improve the dry strength of paper.    (ii) Polymers capable of additionally forming ionic bonds to starch and/or cellulose fibers, such as highly cationic polyvinylamines, can provide dry strength and some permanent wet strength to the paper.    (iii) Polymers capable of covalently bonding to the cellulose fibers, such as glyoxylated polyacrylamide and epichlorohydrin polyamido-polyamines, achieve dry strength and temporary or permanent wet strength of paper.
The cross-linking agents provide wet strength as well as dry strength properties. This material forms a covalent bond with the hydroxyl group on cellulose, and is widely used in applications where wet strength is tolerated and desired.
The wet strength achieved with epichlorohydrin functionalized polymers is of permanent nature, while the wet strength achieved with glyoxylated polyacrylamide is of a temporary nature, losing effectiveness during prolonged exposure to water. This enables re-pulping of broke or waste paper to be readily achieved without special treatment. The dry strength obtained is often greater than that achieved with other conventional strength resins, polyvinyl alcohol, starches or gums.
The glyoxylated polymers typically are less effective in systems where there are high levels of anionics (e.g. anionic trash), such as secondary fiber furnishes. Here the resin complexes with both soluble and insoluble materials, thus reducing the adsorption of the resin onto the fibers. This can be overcome by the addition of cationic promoters (e.g. alumn or polyaluminium chloride), or by careful charge control using other chemical additives in the furnish, such as polyamide wet strength resins or cationic sizes (I. Thorn et al., Applications of Wet-End Paper Chemistry, 2nd edition, Springer, 2009).
Dry and/or wet strength resins are not satisfactory in every respect, particularly because they do not always show optimal performance, particularly in papermaking plants having partially or fully closed water circuits.
Native or chemically modified starch is also extensively utilized in paper manufacture. It has been reported that for production of woodfree uncoated and coated fine papers up to 40 kg starch per ton of paper are applied. Packaging paper made from 100% recovered paper can only be produced economically and in the required quality by adding cost effective biosynthetic starch products. Therefore, these papers are produced with an average starch consumption of 40 kg t−1, mainly by surface application. A further 25 kg t−1 is applied as an adhesive in the converting plant. This means that a high amount of starch is typically returned to the production process via recovered papers, where conventionally it is nearly not retained in the paper sheet. Therefore, this uncontrolled starch quantity leads to a considerable load in the white water circuit (usual COD levels from 5,000 to 30,000 mg O2 I−1) and finally also in the waste water (cf. H Holik, Handbook of paper and board, Wiley-VCH Verlag GmbH & Co. KGaA, 1st ed, 2006, Chapter 3.4.3).
Starch that is released in the wet end of a papermaking machine by the pulping of waste paper or broke is not fixed to fiber except through natural retention and it does not usually contribute to strength parameters.
WO 01/36740 A2 discloses methods of making paper or paperboard. In one method, at least one cellulytic enzyme composition and at least one cationic polymer composition are introduced to a paper making pulp at about the same time to form a treated pulp.
EP 0,361,763 A2 discloses a composition for flocculating paper- or boardmaking filler comprising particles of starch in aqueous suspension, and a flocculating agent, e.g. a polyacrylamide.
WO 2005/042843 A1 discloses a papermaking process, wherein a first strength agent is added to a stock suspension containing pulp and optionally other additives prior to its being formed into a web at the wet end of a papermaking machine.
DE 24 33 325 A1 discloses a process for the manufacture of paper and cardboard from waste paper in closed circuits.
WO 2006/060784 A2 discloses an aqueous printing ink and coating composition containing colorant, one or more high molecular weight starches and one or more water soluble acrylic polymers or co-polymers.
US 2006/289139 A1 discloses a method of improving retention and drainage in a papermaking process. The method provides for the addition of an associative polymer, starch or a starch derivative and optionally a siliceous material to the papermaking slurry.
US 2005/155731 A1 discloses a papermaking process, wherein a first strength agent is added to a stock suspension containing pulp and optionally other additives prior to its being formed into a web at the wet end of a papermaking machine.
WO 2009/059888 A1 discloses fiber products, comprising in their body at least 20% by weight of cellulose fibers, and adequate amounts of an acid and a cationic retention aid for the acid, that can be marked by means of a laser beam.
WO 2006/014426 A1 relates to the manufacture of insulation paper facing having improved reduction or inhibition in the growth of mold and/or fungus.
US 2004/171719 A1 discloses a starch composition that is made by cooking a starch and combining the cooked starch with a polymer, the polymer containing anionic groups or potential anionic groups.
There is a demand for a method for manufacturing paper, paperboard or cardboard which has advantages compared to the methods of the prior art.